grep: Search in Files

A log file can have thousands of lines. You rarely want all of them. You want the lines that mention a particular word. The grep command finds them for you.

grep searches a file and prints only the lines that contain your search text.

The pattern is: grep WORD FILE.

grep error system.log

error: sensor 3 offline
error: low coolant

Every line that contains error is printed; every other line is hidden.

Searching for a phrase

If your search text contains spaces, wrap it in quotes:

grep "low coolant" system.log

When nothing matches

If no line matches, grep simply prints nothing and returns you to the prompt. That is not an error. It means "not found".

grep is one of the most powerful everyday tools on Linux. "Where is this word mentioned?" grep answers it instantly.

Mission: Warning Scan

The station commander wants a quick briefing on the warnings logged during this shift, and a copy of the log filed for the record. The event log is in system.log.

Print only the lines that contain the word warning.
Run a second search for the lines that contain error.
Create a folder called scan and copy system.log into it as shift.log for the record.

Expected result

The warning lines appear, then the error line, and the scan folder holds a copy of the shift log.